The present invention relates to an X-ray fluorescence spectrometer for analyzing sulfur contained in a sample.
The concentration of sulfur contained in fuel oil such as, for example, gasoline, diesel oil or kerosene has its uppermost limit determined by the standards and is, accordingly, measured during or after refinement of the crude oil for administrative or control purpose. In recent years, regulations have become tighter not only in Japan, but also in the United States and European countries and the uppermost limit in the relevant standards has come to be lowered, accompanied by the requirement to analyze even a very small quantity, say, 5 ppm or smaller, of sulfur.
The X-ray fluorescence analysis has hitherto been employed to analyze the sulfur concentration in the fuel oil such as, for example, gasoline, diesel oil or kerosene, but is incapable of providing a sufficient precision of analysis and the lower limit of detection at a concentration lower than the uppermost limit determined by the current standards.
As the conventional X-ray fluorescence spectrometer for analyzing sulfur contained in the fuel oil such as, for example, gasoline, diesel oil or kerosene, which has hitherto been utilized, an X-ray fluorescence spectrometer, including an X-ray tube having a titanium target, a titanium foil filter, and an X-ray fluorescence spectrometer including an X-ray tube having a scandium target and a scandium foil filter, and an X-ray fluorescence spectrometer including an X-ray tube having a scandium target and a titanium foil filter are known. (See the Patent Document 1 below.)
The X-ray fluorescence spectrometer 6 disclosed in the Patent Document 1 is an energy dispersive X-ray fluorescence spectrometer which is, as shown in FIG. 6, so designed that primary X-rays 62 emitted from an X-ray tube 61 having a titanium target are, after having been filtered through a titanium foil filter 63, irradiated to a sample S and fluorescent X-rays 65 emitted from the sample S are measured by a semiconductor detector 68 without being monochromated by any spectroscopic device.
While the primary X-rays 62 before being filtered through the titanium foil filter 63 contain a substantial amount of continuous X-rays together with Ti—Kα lines, the primary X-rays 62 after having been filtered through the titanium foil filter have such an X-ray spectrum as shown in FIG. 8, in which continuous X-rays are appreciably cut off. However, the continuous X-rays are not completely cut off and the remaining continuous X-rays are irradiated to the sample S, causing the latter to generate scattered X-rays which form a considerable background. With the semiconductor detector 68, the energy resolution, that is, sufficient separation of the wavelengths cannot be achieved, and, therefore, the background cannot be sufficiently minimized in comparison with S—Kα line, which are fluorescent X-rays representative of sulfur that have been emitted from the sample. For this reason, the background, when converted into the concentration of sulfur, is as large as 100 ppm and the precision of analysis for a very small amount of sulfur is in no way sufficient to accommodate the recent standard value.
Even the X-ray fluorescent spectrometer including the X-ray tube having the scandium target and the scandium foil filter, and the X-ray fluorescent spectrometer including the X-ray tube having the scandium target and the titanium foil filter, which are the X-ray fluorescent spectrometers disclosed in the Patent Document 1 below, have problems similar to that discussed above and accordingly the energy dispersive X-ray fluorescent spectrometer is incapable of achieving the lower limit of detection and a sufficiently precision even though it makes use of an X-ray tube, having a target suitable to analysis of sulfur, and a primary beam filter.
Also, a wavelength dispersive X-ray fluorescent spectrometer is known, which includes, as shown in FIG. 9, an X-ray tube 91 having a target of rhodium (Rh) capable of generating Rh—Lα lines (2.70 keV) or palladium (Pd) capable of generating Pd—Lα lines (2.84 keV), which are characteristic X-rays having an excellent excitation efficiency to S—Kα line (2.31 keV), which are fluorescent X-ray line of sulfur, in the vicinity of the absorption edge wavelength of sulfur, and a proportional counter 98 utilizing argon gas as a detector gas.
Since this X-ray fluorescence spectrometer 9 does not make use of a primary beam filter disposed on a portion of the path of travel of the X-rays between the X-ray tube 91 and the sample S, the sample S is irradiated with primary X-rays 92 containing a substantial amount of continuous X-rays generated from the X-ray tube 91 and the fluorescent X-rays 97, which have been monochromated by a graphite spectroscopic device 96 to measure the fluorescent X-rays 95 emitted from the sample S, are measured with the proportional counter which is an X-ray detector, but scattered X-rays generated by the continuous X-rays are generated in a substantial quantity and, accordingly, the spectroscopic device 96 is incapable of sufficiently minimizing the background of the S—Kα line, which are analytical lines of sulfur.
If a primary beam filter in the form of an aluminum foil of 12 μm in thickness is disposed on a portion of the path of travel of the X-rays between the X-ray tube 21 of the conventional X-ray fluorescence spectrometer 9 and the sample S, the transmittance for the X-rays with the energy of S—Kα line, which form the background of the S—Kα line, will be 0.6%, but the transmittance of the Ph—Lα line and the Pd—Lα line from the X-ray tube 91, which is a source of excitation of the fluorescent X-rays of sulfur, is 5% or lower and, accordingly, the efficiency of excitation of the sulfur is extremely low and the sulfur, which is an element to be analyzed, cannot be sufficiently excited, making it difficult to achieve the analysis with high sensitivity.
While the conventional wavelength dispersive X-ray fluorescence spectrometer 9 is so designed that the fluorescent X-rays 95 emitted from the sample S are monochromated by the spectroscopic device 96 to select the X-rays of S—Kα line, which are in turn detected by the proportional counter 98, the X-rays diffracted by the spectroscopic device 96 are not only first order line of S—Kα line having an energy of 2.31 keV, but also second order line having an energy of 4.62 keV, and then both diffracted X-rays are incident on the proportional counter 98.
At this time, the X-rays of an energy of 4.62 keV included in the scattered X-rays generated as a result of irradiation of the continuous X-rays from the X-ray tube 91 upon the sample are also diffracted by the spectroscopic device 96 and are incident on the proportional counter 98. When the X-rays of 4.62 keV are incident on the proportional counter 98 having argon gas, the energy of the X-rays is lost by 3.0 keV, which is the energy of Ar—Kα line, due to the argon contained in the proportional counter 98, resulting in appearance of an escape peak of 1.62 keV. This escape peak is equivalent to 70% of the energy of the S—Kα line, which is analytical line of sulfur and cannot be separated by a pulse height analyzer 99 from the S—Kα line, thus constituting a large background.
It has been well understood that the fluorescent X-rays from the sample are incident on the proportional counter and the argon gas, which is a gas in the detector, exhibits an escape peak at the value of energy, which is lower than the incident fluorescent X-rays by an energy of the Ar—Kα line and the fluorescent X-rays of higher order lines such as second order line form interfering lines to the analytical lines. By way of example, Co—Kα third order line interferes the S—Kα lines. However, it has not yet been understood that the higher order lines such as the second order line of the continuous X-rays diffracted by the spectroscopic device result in an escape peak and this forms a background, resulting in error in analysis and deterioration in analytical precision, and this phenomenon has now been discovered. Accordingly, to remove the interference of the higher order lines of the continuous X-rays has not been attempted before.
As discussed above, with the conventional wavelength dispersive X-ray fluorescent spectrometer, sulfur cannot be sufficiently excited if the primary beam filter such as, for example, an aluminum foil is used, and a quantitative analysis of a very small amount of sulfur contained in the fuel oil has been considered difficult to achieve. Also, the second order line of the scattered X-ray of the continuous X-rays from the X-ray tube causes an escape peak attributable to the argon gas in the proportional counter, which in turn interfere the S—Kα line, which is analytical line of sulfur, to thereby form a large background, making it difficult to analyze a very small amount of sulfur with high precision. Also, since the background is high, variation of the background among samples adversely affects the analytical values, resulting in an analytical errors.    [Patent Document 1] JP Laid-open Patent Publication No. H08-136480